cmd/compile/internal/gc: rename Fatal to Fatalf

This helps vet see a real issue:

    cmd/internal/gc$ go vet
    gen.go:1223: unreachable code

Fixes #12106.

Change-Id: I720868b07ae6b6d5a4dc6b238baa8c9c889da6d8
Reviewed-on: https://go-review.googlesource.com/14083
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/swt.go b/src/cmd/compile/internal/gc/swt.go
index f34b1c6..c1a5a6d 100644
--- a/src/cmd/compile/internal/gc/swt.go
+++ b/src/cmd/compile/internal/gc/swt.go
@@ -348,7 +348,7 @@
 		n := l.N
 		setlineno(n)
 		if n.Op != OXCASE {
-			Fatal("casebody %v", Oconv(int(n.Op), 0))
+			Fatalf("casebody %v", Oconv(int(n.Op), 0))
 		}
 		n.Op = OCASE
 		needvar := count(n.List) != 1 || n.List.N.Op == OLITERAL
@@ -679,7 +679,7 @@
 		for _, c := range cc {
 			n := c.node
 			if c.typ != caseKindTypeConst {
-				Fatal("typeSwitch walkCases")
+				Fatalf("typeSwitch walkCases")
 			}
 			a := Nod(OIF, nil, nil)
 			a.Left = Nod(OEQ, s.hashname, Nodintconst(int64(c.hash)))